Heterodyne eliminator



J. l. A. McLAuGHLlN HETERODYNE LIMINATOR Dec. 12, 1944.

3 Sheets-Sheetv 2 '-v Filed Aug. 14, 1941 Awww@ QQN WNNMS @mbv De12f1944 .1.1. A. McLAuGl-ILIN t 2,364,863

HETERODYNE ELIMINYATOR Filed Aug. 14, 1941 I5 Sheets-Sheet 3 Patented Dec. 12, 1944 UNITED STATES PATENT OFFICE HETERODYNE ELIMINATOR James LA. McLaughlin, Detroit, Mich.

Application August 14, 1941, Serial No. 406,883

11 Claims.

This application relates to a heterodyne eliminator, and more particularly to apparatus adapted to eliminate an undesired signal so closely adjacent a desired signal that it lies in one of the sidebands associated with the desired signal and causes highly objectionable heterodyne interference.

One feature of this invention is that it provides improved apparatus for eliminating undesired signals and heterodyne. interference therefrom; another feature of this invention is' that it provides means for changing the frequency characteristics of an undesired signal without changing the frequency characteristics of the desired signal; yet another feature is that either of the two side-bands associated with a desired signal may be rapidly and conveniently selected by switch actuated means, the other sidebandbeing attenuated; still another feature of this invention is that, by the use of two different heterodyne frequencies, undesired signals may be caused to set up an image signal of similar frequency and voltage, the undesired signal and its image neutralizing each other out; a further feature of this invention is that it provides radio receiving apparatus able to cut through and get a desired signal in the face of interference no w rendering such a signal unintelligible; other features and advantages of this invention will be apparent from the following specification and the draw-l ings, in which:

Figure 1 is a schematicblock diagram of apparatus embodying a preferred modification of my invention; Figure 2 is a schematic block diagram of apparatus embodying another modification of my invention; Figure 3 is a circuit diagram of the apparatus shown schematically in Figure 2; and Figure 4 is a partial circuit diagram of apparatus embodying another modification of my invention.

A radio signal comprises three component parts, a, carrier wave of fixed frequency and two sidebands of varying width created by modulation of the carrier wave with the intelligence which it: is desired to transmit. Receiving apparatus can, of course, be designed with almost any desired selective results in chopping o the outer edges of the sidebands and in losing part of the intelligence. Accordingly, the usual radio receiver is provided with a selectivity of about ten kilocycles width, designed to receive the carrier and two thousand cycles not only loses the higher notes of music, but even renders speech relatively unintelligible.

It will be understood that the above discussion is directed to the type of amplitude modulation now generally used conventionally, and my invention will be described in connection with such a system. It will be understood,`however,tthat my invention would also find use with very narrow band radio telegraphy signals, or very wide frequency modulated signals. The principles are the same in each case, and it is thought that it would unduly complicate the disclosure of my invention if an effort were to be made to describe itin connection with every possible system of transmission and reception.

Where an undesired signal carrier lies within a sideband, therefore, the ordinary radio receiving system is unable to eliminate it, and it heterodynes with the desired signal carrier to produce an annoying single frequency tone which substantially or entirely crowds out the desired intelligence. For example, a pair of undesired signal carriers one and one-half kilocycles and two kilocycles lower in frequency then the desired carrier result in the production of three heterodyne beat notes of five hundred cycles,l fteen hundred cycles, and two thousand cycles per second. If the undesired carriers are of substantial amplitude the resultant beat notes will substantially entirely blanket out the desired intelligence.

As I have heretofore disclosed in an article in the.Y June, 1941, issue of QST, I have found that under such conditions selection of the upper sideband and elimination of the lower sideband will entirelyv remove the undesired signals and their heterodyne interference without losing the intelligence, since it has long been known that all of the desired intelligence can be derived from a single sideband, single sideband transmission and reception having been used in certain specialized phases of radio work, as transoceanic radio telephony. Even where undesired signals are presamount of selectivity,but making the circuits too 5 ent in both sidebands, as where there are two undesired signals in the lower sideband and one in the upper sideband (resulting in six distinct heterodyne beat notes), selection of the upper sideband and elimination of the lower results in elimination of iive of the six distinct heterodyne interference notes.

In carrying out my invention my preferred apparatus comprises switch actuated means whereby the receiver may be operated as a conventional receiver or as a single sideband receiver;v and in the latter case, either the upper or lower sideband may be selected by a simple one a certain frequency above that of the desired' signal and the other an `equal frequency below, and causing one or the other oscillator to heterodyne with the input which contains both the desired and undesired signal. In conjunction with this arrangement I employ a band pass filter cutting E sharply below the changed frequency of lthe desired signal, and passing four or five kilocycles of frequencies above that of the changed desired signal.

The use of two such different heterodyning oscillators enables the elimination of two completely separate single band pass amplifiers, so that either sideband may be readily chosen with only a single band pass amplifier. It will be understood that this arrangement is applicable to single sideband transmitters as well as to receivers, although it is here being disclosed only in .connection with a receiver.

I have also found that the use of two separate oscillators having frequencies spaced equally above and below the frequency of the desired signal enables creation oi what I term an image of the undesired signal. That is, if the desired signal has a frequency of 450 kilocycles and the undesired signal'has a frequency of 451 kilocycles, heterodyning of i a portion of this input with a 500 kilocycleoscillator will result in an output comprising carriers of 50 and 49 kilocycles, while heterodyning of another portion with an oscillator vhaving afrequency of 400 kilocycles will result in an output containing frequencies of 50 and 51 kilocycles. That is, the undesired signal, which may be considered as having come through as 51 kilocycles, has had an image created at 49 kilocycles. I have taken advantage of this to provide a system wherein the undesired signal and its image is duplicated by another undesired signal and image of equal voltage but opposite phase,

so that the combination of these neutralizes out the undesired signal and its image. By a particular arrangement which I will hereafter more fully disclose I effect this neutralizing out vor balancing out of the undesired signal without losing the desired signal. This embodiment of my invention .has the Afurther advantage of op-l erating automatically, so that any undesired signaisA neutralize themselves out, when the desired signal is properly tuned in, without particular adjustments by the operator, in this regard being a great step ahead of so-called innite rejection circuits which require careful and preciseV tuning of the rejection slot to the undesired frequency,' and which even then are only able to remove one or two undesired frequencies.

In the particular embodiment of my invention schematically illustrated in Figure 1 it will .be understood that the signal delivered to the amplifler here termed the high frequency amplifier is a desired signal at a conventional 'intermediate frequency. 'I'hat is, my apparatus as disclosed in Figure 1 would be preceded by a conventional radio frequency amplifier, preferably tuned, a tuning oscillator and a mixer, so that the desired'signal would be changed to a frequencyintended for usual intermediate frequency use, as 455 kilocycles per second. In a complete receiver embodying my invention there are really -tWo stages or intermediate frequencies through which the radio frequency signal passes before it is rectified toprovide the audio frequency intelligence, and I am terming the rst frequency the high frequency, it being understood that this is lower than the original signal frequency .but higher than the second intermediate frequency.

By adjustment of the tuning oscillator, therefore, the desired signal has been converted to a frequency of 455 kilocycles, this being illustrated as the solid black vertical line in the center of the high frequency amplifier. This signal has two side bands four or five thousand cycles in widthassociated with it, these being illustrated as the rectangles lying on each side of the 455 k. c. carrier. An undesired carrier frequency may have come through as a frequency of 452 k. c.,1y ing in the lower sideband and being indicated by the vertical dotted line.

I make use of two crystal oscillators adapted to oscillate precisely at a certain frequency above and the same frequency below that of the desired signal. In Figure 1 I have indicated one of these crystal oscillators as A, this oscillator operating at 405 k. c.; and the other as crystal oscillator B, this operating at 505 k. c. Since both differ in frequency exactly 50 k. c. from the frequency of the desired signal, heterodyning of the output of either of these oscillators wlth'the desired signal will result in a changed desired signal of 50 k c., illustrated by the solid vertical line in the center of the mixer. This changed signal will, of course, still have its side bands with it, as indicated. Theundesired signal, however, can now be made to `lie in either sideband, depending upon the oscillator used. If'oscillator'A is supplied to the mixer for the heterodyne operation the undesired signal will have a frequency of 47 k. c., as indicated by the dotted line to the left; while if oscillator B is used the undesired signal will have a frequency of 53 k. c., as indicated by the dotted line to the right.

The output of the mixer is divided into two portions, one being supplied to a vband pass amplifier adapted to pass a frequency band from 50 to 54 k. c., and the other portion being supplied to a sharply tuned tuning meter amplifier, the output of this amplier being rectified by a de- Y tector and supplied to a resonance meter to indi- 50 cate correct tuning of the tuning oscillator of the receiver. It will be understood that the selectivity characteristic curve of the tuning meter amplifier will be as sharply peaked as possible to 50 k. c., preferably being 60 db. down at 2 k. c. off resonance either side. The band pass amplifier selectivity, on the other hand, is arranged to cut olf sharply below 50 k. c., but to practically uniformly accept frequencies up to 54 k. c., and then to again drop ofalthough there is no necessity for this side of the selectivity characteristic curve being as sharp as the other side. The requirement for only one sharp side, of course, enables the band pass amplifier to be more sharply and simply constructed than would be vpossible if it were desired to hold a flat band 4 k. c. wide with'a sharp drop ofi` on both sides.

' In an embodiment of my invention which I have circuit practice.

being operated as a standard double sideband receiver, the 452 k. c. undesired signal would result in a highly objectionable heterodyne interference note which could not be readily eliminated by the present conventional methods.'

Without even knowing the exact location of the undesired signalfor whether it is above or below the desired signal, it is only necessary for an operator using my receiver to throw the oscillator switch to one side for a second or two, and then to the other s ide,`to determine which eliminates the heterodyne interference. In the assumed conditions the operator will instantly find that use of the oscillator B would result in heterodyne interference (the 53 k. c. undesired signal lying -in the sideband passed by the band pass amplifier) while use of oscillator A would result in complete elimination of the interference (the undesired 47 k. c. signal lying at a point sufficiently below the desired 50 k. c. carrier to be substantially entirely wiped out during passage of the signals4 through the band pass amplifier). That is, with asimple, convenient fiip of a double throw switch from one side to the 'other the operator can instantly determine which setting results in elimination of the heterodyne interference, and can eliminate undesired signals and associated interference which has heretofore been considered impossible to do. In an embodiment of my invention which I have built I make the oscillator switch a three-position'switch, the center position enabling operation of a standard receiver as a double band pass receiver, movement of the switch to one side or the other operating a relay which opens connections in the standard circuit and which diverts the output of the 455 k. c. intermediate frequency amplifier to the input of the high frequency amplifier of my newV apparatus.

I have not illustrated the circuits of the vari-y ous portions of a receiver operating in the manner just described, since it is to be understood that my invention lies in a novel arrangement and use of fairly conventional parts." That is, my apparatus shown schematically in Figure 1 would be preceded by the radio frequency amplifier, tuning oscillator and mixer` of a conventional receiver, preferably of the communications type; my high frequency amplifier would be built'in accordance with standard intermediate frequency amplifier practice; my oscillators would each in themselves be conventional fixed oscillators, although precision controlled to the desired frequency; the mixer would, of course, follow conventional practice, as would also the tuning meter amplifier and its associated detector and meter; the band pass amplifier would be vconstructed in accordance with conventional single side-band practice, except that it need be designed only to sharply attenuate one side of the desired band; and the detector and audio frequency amplifier would also follow conventional In Figure 2 I have shown schematically another embodiment of my invention which serves to eliminate undesired signals closely adjacent the desired signal, `this modification automatically eliminating undesired signals either above or below the desired signal, or both, without switching or other attention by the opera-tor of the receiver. In order that there may be no confusion between the description of this embodiment and that of the previously described embodiment, I am here assuming that the desired signal has been converted, by the tuning oscillator and mixer, to a frequency of 450 k. c., and that an undesired closely adjacentsignal then has a frequency of 451 k. c. ,Again these signals would be supplied tothe novel portion of my apparatus illustrated in the drawing by a conventional radio frequency amplifier, tuning oscillator, and mixer, preferably being tapped'off from the standard receiver just after the intermediate frequency amplifier, which we will assume was tuned to 450 k. c. This may be considered as delivered to another 450 k. c. amplifier, identified in Figure 2 as a high frequency amplifier, .the output of this amplifier being divided into three portions and delivered to three mixers. 'I'he upper and lower mixers, here identified as heterodyne mixers, are of the simple singletube type, while the center mixer isa duplex or push-pull mixer.

I again employ two oscillators, identified as oscillators A and B having frequencies a certain frequency above and below 450 k. c.,` in this case being indicated as oscillators'of 400 k. c. and 500 k. c., respectively. The outputs o'f these amplifiers are connected through phase Shifters, to enable control of the phase of the oscillator voltage, to the mixers. The output of the phase shifter of oscillator -A ls connected to the upper heterodyne mixer and the upper tube of the duplex mixer; while the output of the phase shifter associated with oscillator B is connected to the lower heterodyne mixer and the lower tube of the duplex mixer.

The output of the duplex mixer, as is indicated in Figure 2, contains the desired changed fiftykilocycle signal, and the undesired signal and an image of it, these being indicated at 49 and 51 k. c; frequencies. As a result of the arrangement of the output transformer of this push-pull or duplex mixer,` all of these frequencies are of equal voltage. The upper heterodyne mixer outfrequencies.

put, on the other hand, contains only the desired 50 andthe undesired 51 kilocycle signals, and that of the lower heterodyne mixer comprises the 50 and the 49 kilocycle frequencies. are to provide an output also comprising 49, 50 and 51 kilocycle frequencies, but in this case it will be noted that the 50 kilocycle frequency has substantially twice the voltage ofthe undesired These two outputs are delivered to a push-pull 50-kilocycle frequency amplifier, and as a result of adjustment of the phases and voltages the outputs are arranged so that the undesired signals have equal voltages opposite in phase and cancel each other out, leaving only the desired signal of an amplitude determined by the difference in voltage of the desired signal in the two 'mixer outputs. vThis desired signal, after passing through the push-pull intermediate frequency amplieris rectified by a detector and amplified by an audio frequency amplifier to provide the desired audio output, completely free (or substantially so) of the undesired signals.

Inasmuch as some of the circuit arrangements associated with this embodiment of my device are different from what has heretofore been used,

I have shown in Figure 3 a circuit diagram of the apparatus schematically illustrated in Figure 2. The incoming signals are supplied to the primary of transformer I0 and amplified by passing through the tubes'II and I 2. amplifying tube I2 acts as a blocker, the input signalsv being divided into three portions at the point I3. One portion goes to the tube I2, another portionto the blocker or radio frequency amplifying tube I4, and the third portion to another' blocker amplifyingV tube I5. The outputs These The Second ,addition or mixing of these two outputs. phases have been properly chosen, and the yvoltof these various blocker 'tubes are connected to the mixers, the tube I4 feeding into the simpley tube I1. The tube I2, on the other hand, has

its output supplied to the two mixer tubes I8 and .I8 arranged in push-pull to provide a duplex mixer.

The tube 20 and its associated c ircuit, including the control crystal 2|. provides oscillator A; while the tube 22 with its circuit and control crystal 23 acts as oscillator B. Both of these oscillators feed into phase shifting networks here indicated in general as 24 and 25. The output of the phase shifting network 24 is applied to separate grids in the tubes I6 and I8, while the output of the phase-shifting network 25 is applied to grids in the tubes I1 and I9. 'Ihe outputs of the tubes I6 and I1 are both developed across the primary of the transformer 26, shunted with a variable resistor 21 to enable regulation of the voltage.

The output of the duplex mixer including the tubes I 8 and I9 is developed across the primary of a push-pull transformer 28. This transformer has a ratio of one to one between its total primary turns and its secondary turns and the primary is center tapped. In so far as the 51- kilocycle frequency is concerned, this exists only in the tube I8 and is developed across only half of the primary. The transformer 28 is'preferably of relatively low impedance, much lower than the plate impedance of the tube working into it, and under such conditions the 51kilocycle output of the tube I8 will pass through the transformer with a two to one step-up ratio. The same is true of the 49-kilocycle undesired signal image in the tube I9, this being developed in half of the transformer only and being stepped up to create double voltage in the secondary of the transformer 28. The 50-kilocycle signal, on the other hand, appearsin both tubes and these tubes act in regular push-pull relation. Thus, although there is twice the signal voltage present, the 50-kilocycle voltage generated in the secondary is the same as that of the undesired signals, since as to the desired signal there is only a one to one ratio rather than a one to two ratio. The

combined output of the tubes I6 and I1, on the other hand, gives a signal wherein the undesired signals have equal voltage but the desired 50- kilocycle signal has. a greater voltage, double if the outputs are in proper phase. 1

Under such circumstances the two transformers 26 and 28 working into the push-pull ampliiler comprising the tubes 29 and 30 cause an If the age is properly adjusted by the resistance 21, the undesired 49 and 51 kilocycle signals will neutralize each other out, and only the desired 50-kilocycle signal will appear in the output transformer 3|, since the voltages of this desired signal were not equal and even though they were opposite inA phase'this signal will not cancel out. The output of the transformer 3|, comprising only the desired signal, is then passed through a single-ended amplifier which may comprise only the tube 32, or a series of such tubes in cascade, rectified by the detector tube 33, am-

plified in an audio amplifier comprising one or more tubes such as 34, and delivered to any desired output source, as the earphones 35.

Referring now more particularly to Figure 4, I have illustrated a somewhat different modification of the embodiment of my invention illustrated in Figure 2. In Figure 4 the 'upper tube 40 vmay be considered to represent the upper heterodyne mixer; the tubes 4I and 42 comprise part of the duplex or push-pull mixer; and the tube 43 comprises the lower heterocwne mixer, the parts preceding these tubes not being shown, since they would duplicate the circuit arrangement of Figure 3.

In this form of my invention I do not neutralize out the undesired signal images of each side'ofthe desired signal simultaneously, but eect this in two different steps. It will be noted that the output of the transformer 44 contains frequencies of 49, 50 and 51 kilocycles per second, of equal voltage, as explained before in connection with the output ofthe push-pull mixer. The output of the tube 43, on the other hand, delivered through the transformer 45, contains -only the 50 and 49 k. c. frequencies, so that when these two outputs are combined in the push-pull amplifier comprising the tubes 46 and 41 proper adjustment of the relative voltages and phases effects complete elimination of the 49 k. c. un-

desired signal, at the same. time eliminating the v 50-kilocycle signal. Thus the output of the transformer 48 would contain only a 51 k. c. signal. The output of the upper heterodyne mixer comprising the tube 40, on the other hand, comprises 50 and 5l k. c. frequencies, and when these are mixed with the 5I k. c. output in the push-pull amplifier comprising the tubes 49 and 50 proper adjustment of the phases and voltages will result in complete elimination of the 51 k. c; signal (together with its sidebands,of course) in the output of the transformer 5I, which would then be supplied to any conventional audio amplifler after having been recti/ed.

The advantage of this latter form ofv my invention is that it provides greater flexibility of adjustment, enabling the signalsto always be properly balanced out. If necessary, additional phase-shifting networks can be inserted in one or the other of the simple heterodyne mixer outputs so that, together with the voltage regulat.. ing resistors 52 and 53, complete control may be had of the various frequencies. As before, the

"conversion of the undesired signal into a similar Changes, therefore, in the construction andarrangement may be made without departing from the spirit and scope of the invention as disclosed Vin the appended claims.

Iclaim: l. Apparatus of the character described, in-

cluding: means for providing a desired signal ing a frequency equal to thatof the desired signal less the intermediate frequency; 'and means for selectively causing one or ther other of said oscillators to heterodyne with the desired signal to select which. of said sidebands is to be passed bythe filter circuit.

odyning said portions with said waves;` and" lators, one having a frequency equal to that of the desired signalv plus the intermediate frequency and the other having a frequency equal to vthat of the desired signal less the intermediate frequency; means for selectively causing one or the other of said oscillators to heterodyne with the desired signal to select which of said sidebands is to be passed by the filtercircuit; and means for rectifying the sideband passed by said filter' circuit.

3. Radio receiving apparatus of the character described adapted to eliminate an undesired signal closely adjacent a desired signal having sidebands associated therewith, including: means for providing a desired signal having sidebands associated therewith; two oscillators, one having a frequency equal to that of the desired signal plus a certain frequency and the other having a frequency equal to that of the desired signal less said certain frequency, whereby heterodyning of said oscillators with the desired signal changes the frequency of the desired signal to the same frequency in both cases while changing that of the undesired signal to different frequencies; and means for eliminating the changed undesired signalv while retaining the vchanged` desired signal. 4. Radio receiving apparatus of the character described adapted to eliminate an undesired. signal closely adjacent a desired signal having sidebands associated therewith, including: means for providing a, desired signal having sidebands associated therewith; twol oscillatorsl one having a frequency equal to that of the desired signal plus a certain frequency and the other having a frequency equal to that of the desired signal less said certain frequency', whereby heterodyning of said oscillators with the desired signal changes theV frequency of the desired signal to the same frequency in both cases while changing that of the undesired signal to different frequencies; a band pass filter circuit adapted to pass one of the sidebands associated with the changed desired signal and exclude the other; and means for selectively causing one of or the other of said oscillators to heterodyne with the desired signal to select which of said sidebands is to be passed by the filter circuit.

5. Apparatus of the character claimed in claim 4. wherein 'said last mentioned means is'switch actuated. l

6. Radio receiving apparatus of the character described adapted tov eliminate an, undesired signal closely adjacent a desired signal having sidebands associated therewith, including: means for providing a desired signal having sidebands associated therewith; means for providing two separate waves, one having a frequency equal to that of the desired signal plus a certain frequency and the other having a frequency equal to that of the desired signal less said certain frequency, vwhereby heterodyning of said Waves with the desired signal changes the frequency of the desired signal to the same frequency in both cases while changing'that of the undesired signal to diiferent frequencies; means for dividing the signal into portions and heterthe undesired signals to neutralize each other. 7. Radio receiving apparatus of the character y waves, one having a. frequency equal to that of means for combining the heterodyne. outputs in such phase and voltage relation as to cause described adapted to eliminate an undesired signal. closely adjacent a desired signal hav-- ing sidebands associated therewith, including: means for providing a desired signal having sidebands associated therewith; two oscillators, one having a frequency equal to that of the desired signal plus a certain frequency and the other having a frequency equal to that of the desired signal less said certain frequency, whereby heterodyning of said oscillators withv the desiredA signal changes thefrequency of the desired signal to the same frequency in'both cases while changing that of the undesired signal to diiferent frequencies; means for dividing the signals into portions and heterodyning said portions with said oscillators; means whereby one heterodyne output comprises all of the changed. signals with the mean voltages of the undesired changed signals -being equal and that of the desired changed signal being different; means whereby another heterodyne output comp'rises all of the changed signals with equal mean voltages; and means for combining the heterodyne outputs in such phase and voltage vrelation as to cause the undesired signals to neutralize each other.

8. YApparatus of the character described,l including: means for'providing a desired signal having sidebands associated therewith; an intermediate frequency band pass lter circuit adapted to pass one of said sidebands and exclude the other; means for providing two separate the desired'signal plus the intermediate freing sidebands associated therewith, including:`

means for providing a desired signal having sidebands associated therewith; means for prodesired viding two separate waves of xed frequency,

one'having a frequencyequal to that of thev desired signal plus a certain frequency'and the other having a frequency equal to that of the signal I, less said' certain frequency, whereby heterodyning of said waves with the desired signal changes the frequency of the desired signal .to thesame frequency in both cases while changing that of the undesired signal to different frequencies; and means for eliminating the changed undesired signal while retaining the changed desired signal.

10. Radio receiving apparatus of the character describedadapted to eliminate an undesired signal closely adjacent a desired signal having upper land lower sidebands associated therewith,

including: tunable means for converting the desired signal to a predetermined frequency, whereby the sidebands are correspondingly converted; means for providing two separate waves of fixed frequency, one having a frequency equal to said predeterminedfrequency plus a certain frequency and the other having a frequency equal to said predetermined frequency less said certain frequency, whereby'lheterodyning of said waves with the converted desired signal changes its frequency to the same frequency in both cases while changing that of the converted undesired signal to different' frequencies; and means for eliminating the changed undesired signal while retaining the changed desired signal.

l1. Radio receiving apparatus of the characber described adapted to eliminate an undesired signal closely adjacent a desired signal having upper and lower sidebands associated therewith,

including: turnable means for converting the desired signal to a predetermined frequency, whereby the sidebandsare correspondingly converted; a, band pass filter circuit adapted to pass one of said converted sidebands and exclude the other; and switch-actuated means, separate from said tunable means, for selectively reversing the order of the sidebands, -Whereby one or the other of said sidebands lies within the pass band of the illter circuit.

'JAMES L. A. McLAUGHim. 

